🚀 go-pugleaf

On Wed, 11 Jan 2012 08:30:48 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>John Larkin wrote:
>> On Tue, 10 Jan 2012 15:39:08 -0800 (PST), Bill Sloman
>> <bill.sloman@ieee.org> wrote:
>> 
>>> On Jan 10, 5:50 pm, John Larkin
>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>>>> On Tue, 10 Jan 2012 17:29:35 +0100, Jeroen Belleman
>>>> <jer...@nospam.please> wrote:
>>>>> John Larkin wrote:
>>>>>> On Tue, 10 Jan 2012 01:28:22 -0800 (PST), Bill Sloman
>>>>>> <bill.slo...@ieee.org> wrote:
>>>>>>> On Jan 10, 7:47 am, "David L. Jones" <altz...@gmail.com> wrote:
>>>>>>>> Just thought some might be interested in hearing our recent guest on
>>>>>>>> our Amp Hour radio show, Dr Howard Johnson of "Handbook of Black
>>>>>>>> Magic" fame[...]
>>>>>> Yeah, his book is half nonsense. If you can tell which half is which,
>>>>>> you don't need the book. His ideas on "return currents" and bypassing
>>>>>> are absurd. And the writing is dreadful.
>>>>> I haven't got the book handy, but I'll pick it up tomorrow or so.
>>>>> Do you mind expounding on why his "return currents" make no sense?
>>>>> Did he mean something different from image currents?
>>>> He says that a transmission-line pcb trace must always be "referenced"
>>>> to the same "return current" plane it started with and shouldn't pass
>>>> through vias to become referenced to a different plane, or else the
>>>> return currents will get confused or go on strike or something. If
>>>> that were true, 98% or the multilayer boards on Earth would quit
>>>> working.
>>>>
>>>> He also says that you should add bypass caps and stitching vias to
>>>> conduct these return currents whenever a trace does via to a different
>>>> "reference plane."
>>>>
>>>> All silly.
>>> Not completely silly. One of the more avoidable hold-ups in the
>>> development of the electron beam tester at Cambridge Instruments from
>>> 1988 to 1991 was when the drafting office sent out a six layer board
>>> for manufacture without specifying the order of the inner planes -[...]
>>> The next board had the inner planes in the right order, amongst other
>>> changes, and worked a lot better.
>>>
>>> --
>>> Bill Sloman, Nijmegen
>> 
>> 
>> Of course the stackup and order matter, if not to get the DC
>> connections right, then to get the impedances right. But the obsession
>> with return currents and "reference planes" is mostly silly.
>> 
>> As is the admonition to never cross slits in planes, fr'instance
>> running a trace above a plane that has multiple pour pours.
>> 
>> John
>
>A track across a slit is an antenna. Unless there's another plane
>underneath covering the slit, in which case it probably doesn't
>matter.

On a multilyer board, there usually is. But I've done test slits on
microstrips on 2-sided boards, clear across the board, and not much
happens. I didn't measure radiated EMI, but signal integrity wasn't
much affected at 30 ps risetimes.

John